Poster at the 40th annual meeting of the Biopysical Society . Abstract: Biophys. J. 70, A334 (1996). The traditional methods of probing molecular mobility on membranes, fluorescence photobleaching recovery (FPR) and single particle tracking (SPT), are incapable of providing direct information on the local forces affecting the particles. FPR provides only ensemble averaged information, and SPT yields limited information on the localized membrane properties. To measure local interaction restraints and active forces, we use an optical trap consisting of a strongly focused infrared laser combined with a simple position sensing device and computer controlled stage, providing both picoNewton-level tensiometry and nanometer-scale position detection. This combination of optical force microscope (OFM) and SPT system provides the additional capacity for dynamical, continuous force measurements on the tracked particle. Such a configuration requires the use of tunable feedback to modify the trap position in response to displacements of the tracked particle; our analysis indicates that by varying the feedback transfer function one may also obtain information about the local constraints on particle motion, making this a potentially powerful tool for the investigation of anomalous diffusion on cell membranes. We present a theoretical analysis of the necessary feedback scheme, experimental details of its implementation, and some preliminary results. [NIH (RR04224) and NSF (BIR9419978)]